1. Field of the Invention
The present invention relates to a calibration apparatus for adjusting to prescribed values the emission brightness of light emitting elements, such as light emitting diodes, in an optical printer.
2. Description of the Prior Art
An optical printer selectively irradiates a photosensitive body, such as photographic film, with light from the light emitting elements thereof and forms images on the photosensitive body.
Prior to shipment of an optical apparatus, such as an optical printer using light sources comprising red (R), green (G), and blue (B) LEDs, the output voltage or current of the circuit for driving and controlling the quantity of light is usually set to an appropriate value so that these LEDs emit a prescribed quantity of light. In other words, an optical apparatus is shipped out after the calibration of the current value at which color balance is achieved for each R, G, and B LED, for example, so that the R becomes 5 milliamperes, G becomes 30 milliamperes, and B becomes 10 milliamperes.
Such a pre-shipment calibration is carried out by sampling light from a light source under certain conditions with a phototransistor established outside the optical apparatus, converting the sampled light to a voltage value, comparing the converted voltage value to a reference voltage value, and adjusting the output voltage or current of the circuit for driving and controlling the quantity of light so that both values match.
However, such a conventional calibration brings about variations in error due to noise because the photodiode samples an instantaneous value of light from the light source. Moreover, it is not necessarily possible to ensure that the properties of the light from the sampled light source match the properties of light from the light source when the optical printer is actually operating. For these reasons, reliable calibration has not been possible before now.
It is an object of the present invention to provide a highly reliable calibration apparatus for light emitting elements of an optical printer which is not affected by external noise and which performs calibration in accordance with the actual operations of an optical printer.
The light emitting elements to be calibrated with the calibration apparatus of the present invention are those of an optical printer which forms images on a photosensitive body by selective irradiation of a photosensitive body with light from the light emitting elements. The light emitting elements include photosensors for outputting changes in the emission brightness of the light emitting elements as electrical signals. The voltage value of the electrical signals from these photosensors is integrated over a prescribed period. The resulting value is compared with a reference voltage value and the emission brightness of the light emitting elements is adjusted accordingly.
Furthermore, the optical printer is the line scanning type which forms images on the photosensitive body by irradiating each line with light and scanning a prescribed number of lines. The optical printer comprises a liquid crystal shutter which is controlled to pass or block light from each of the pixels on a line individually. Calibration is carried out by the photosensor detecting light transmitted by the liquid crystal shutter.
Also, the optical printer is constituted to be able to form images having a prescribed number of gradations on the photosensitive body by performing gradation control for each pixel by controlling the length of time the liquid crystal shutter allows passage of the light corresponding to each pixel.
Calibration is performed by detecting a light of which graduation control by the liquid crystal shutter is done at about the middle of the prescribed number of gradations.
Furthermore, calibration is performed by detecting light once the emission brightness of the light emitting elements, which varies from line to line, becomes constant.
Furthermore, calibration is performed by detecting a light of a line which is to be at about the middle of a prescribed number of lines drived.
The calibration apparatus for light emitting elements of an optical printer, according to the present invention, has the abovementioned structure and therefore has the following effects.
1. SN ratio of the calibration apparatus can be high, without the photosensor output voltage varying greatly due to external noise light or external noise voltage, by sampling a value obtained by integrating over a prescribed time an output signal of the photosensor which detects light from the light emitting elements to output the detected value as an electrical signal.
2. Exposure light of the prescribed strength can be attained for all gradation regions, by calibrating the light emitting elements by detecting light which passed through the liquid crystal shutter with the photosensor and detecting a light for which the liquid crystal shutter controls its gradation at half the maximum gradation.
3. Exposure light of the prescribed strength can be attained for all regions of all scanning lines, by detecting the light once the brightness due to light emission for each line of light emitting elements becomes constant.